The present invention relates to rear projection screens and, more particularly, to a new and improved rear projection screen providing increased image contrast.
Various types of rear projection screens for television systems and the like have been proposed, and some of them have already been put into practical use. Rear projection screens for television systems providing a large image are employed mainly to permit a large number of viewers to view the screen at the same time. For this purpose, it is necessary to employ a screen having a greater than normal angular field of vision. Accordingly, a lenticular screen is preferably employed.
A lenticular screen is formed by arranging on at least one surface thereof a multiplicity of elongated lenses of cylindrical shape, for example, with their longitudinal axes placed substantially parallel to each other. In rear projection television systems the lenticular screen is arranged so that the light rays from the projector of a television system tube are received on the side facing the projector, then refracted and condensed by the elongated lenses are thereafter diffused so as to emanate in a specific angular range from the viewing side of the screen. Since the lenticular screen has a multiplicity of elongated lenses arranged on at least one of its surfaces, the front surface of the screen has elongated bright portions where the light is allowed to pass by the refractive condenser effect of the elongated lenses, spaced by dark portions where no light passes. More specifically, the lenticular screen produces on its front surface a multiplicity of parallel bright stripe portions and dark stripe portions. Generally speaking, a rear projection screen having one lenticular surface has on its viewing side surface bright portions where light is transmitted and dark portions where no light is transmitted.
It is preferable that a television projection screen should be visible in a "bright room" having a substantial level of ambient light. If ambient light is incident on an image portion of the television projection screen, however, the dark portion, or black portions, of an image is illuminated, reducing the image contrast.
The image contrast of a television projection screen is generally defined as follows: ##EQU1## where B.sub.MAX is the brightness of the bright region of image; B.sub.MIN is the brightness of the dark region of image; B.sub.1 is the brightness of reflected ambient light at the bright image region; and B.sub.2 is the brightness of reflected ambient light at the dark image region.
In a dark room having no ambient light, the higher the ratio, B.sub.MAX /B.sub.MIN, the higher is the apparent image contrast. In a bright room having substantial ambient light, however, the brightnesses (B.sub.1, B.sub.2) of reflected ambient light must be taken into consideration.
More specifically, in order to increase the image contrast on the television projection screen, it is necessary to keep the brightness of the reflected ambient light at the light and dark image regions as low as possible. To accomplish this at a given level of ambient light, the amount of ambient light absorbed by the television projection screen must be increased. For this purpose, light absorbing elements must be provided on the surface of the screen.
Techniques for providing light absorbing elements on a screen are proposed, for example, in U.S. Pat. Nos. 3,523,717, 3,830,556 and 4,172,219 as well as Japanese Patent Publication No. 46693/77. These patents disclose rear projection screens having a lenticular lens formed on one surface by a multiplicity of cylindrical lenticules and also having shading stripes on the portions of the opposite screen surface in the regions where no light passes in order to increase the image contrast.
In the television projection screen shown in the specification of Japanese Patent Publication No. 46693/77, for example, a photoresist film is coated on the surface of the lenticular screen opposite to the surface provided with the lenticular lens. The photoresist film is exposed, developed and dried to form such a stripe pattern where the exposed portions of the film are left on the surface. These film stripes are located in the region where no light is transmitted by the lenticular surface, and an oil ink is applied to the stripe pattern by offset printing or the like to form light absorbing stripes.
To provide a stripe pattern with oil ink as described above, however, requires a very high level of printing technology and complicated processing is required because the stripes of the stripe pattern are exceedingly fine and accurate positioning is necessary during application of the ink. Moreover, it is necessary to expose, develop and dry the photoresist film, therefore requiring that the manufacture of the television projection screen be carried out in a darkroom. In addition, since the photoresist film has poor adhesion, the processing must be carried out with exceptional care. Accordingly, the manufacturing cost of such screens inevitably increases.
Moreover, the photoresist film method requires that ink be printed or applied directly onto the portions of the television projection screen where no light passes. By such application, however, unevenness of printing is easily produced, and it is difficult to effect a striping which is very narrow and uniform in width as mentioned above.